Method for heat treatment of substrates

ABSTRACT

A method for the heat treating of substrates in which the substrate is passed through a passage surrounded by a gas impervious envelope and in which a high frequency electrical signal is applied to an electrode exteriorly to the gas impervious envelope and to the substrate so that a gaseous plasma is generated within the envelope but not within the central passage.

United States Patent 1191 Boom 1 1 Mar. 18, 1975 METHOD FOR HEATTREATMENT OF 3.090.737 5/1963 Swzlrtz 219/1081 x 3,146,336 8/1964Whitucre 219 121 P SUBSTRATES I 3,182,982 5/1965 Ruff 219/155 XInventor: Abraham Boom, Martmsvllle, 3,203,768 8/1965 Tiller ct 111.219/1043 X NJ. 3,383,163 5/1968 Menilshi 219/121 P X 3,405,301 10/1968H1 ilkllwil et a1. 315 111 X [73] Asslgneei Celanese Corlmmtmn, New York3,571,551 3 1971 OQZSZIWLII'U et 111 219/1061 X 3,572,286 3/1971FOI'I'ICy 219/1061 X 3,636,300 l/1972 Gunnell ct 211. 219/121 P [22]1973 3,671,195 6/1972 Bersin 315/111 X [21] Appl. No.: 389,502

Related us. Application 1361a 2'1"? gf 'l f c g fl [62] Division Of Ser.N6. 185,014, Sept. 30, 1971, Pat. eterso [57] ABSTRACT [52] US. Cl219/121 P, 315/204 A method f heat treating f substrates in which [5111111. C1. 823k 9/00 the Substrate i passed through a passage Surrounded[58 Fleld 0f Search 219/121 P, 121 R, 155, by a gas impervious envelopeand in which a high 219/1043 10611 10811 204; quency electrical signalis applied to an electrode ex- 315/111, 108; 313/158, 157, 162, 3teriorly to the gas impervious envelope and to the substrate so that agaseous plasma is generated within the 156] References C'ted envelopebut not within the central passage.

UNlTED STATES PATENTS 6 Cl 5 D F 2,282,317 5/1942 136116611 219/155 Xraw'ng R.F. SOURCE PATENTEDNARI 8 1975 RF. SOURCE METHOD FOR HEATTREATMENT OF SUBSTRATES This is a division of application Ser. No.185,014, filed Sept. 30, 1971-, now US. Pat. No. 3,780,255.

BACKGROUND OF THE INVENTION The present invention relates to a methodand apparatus for highly concentrated electrical heating and morespecifically to a method and apparatus for efficiently heating asubstrate through the generation of a plasma in the vicinity of thesubstrate.

It is often desirable to heat various substrates at elevatedtemperatures to obtain desired substrate characteristics or to aid inthe coating of the substrate. For example, in the manufacture ofcarbonaceous fibrous materials, carbon graphite fibers may be treated atelevated temperatures to modify the surface or overall characteristicsof the fiber. I

In the past, substrates have been heated in various manners to providethe desired modification of the substrate characteristics. For example,resistance heating, i.e., passing an electrical current through thefiber, has been frequently used to obtain the elevated temperaturesrequired. However, the current flow and therefore the cost of heatingfibers by resistance heating may necessarily be excessively high inorder to reach the temperatures required.

Other conventional electrical methods for heating substances may includeindirect heating through the use of resistively or inductively heatedelements in an oven or other enclosed or semi-enclosed space. Theefficiency of these methods may also suffer due to the necessity ofheating the element from which heat is transferred to the substrate.

It is accordingly an object of the present invention to provide a novelmethod and apparatus for electrically generating high temperatures.

It is another object of the present invention to provide a novel methodand apparatus for generating high temperatures in a relatively confinedheating zone for the treatment of substrates. It is a further object ofthe present invention to provide a novel method and apparatus forelectrically heat treating substrates wherein the substrate is heatedthrough a combination of direct and indirect heating, for example,radiantly, resistively, inductively and through conduction.

It is yet another object of the present invention to provide a novelbalun output transformer structure for selectively coupling RF power tothe heating chamber.

These and other objects and advantages of the present invention willbecome apparent to one skilled in the art to which this inventionpertains from a perusal of the following detailed description when readin conjunction with the appended drawings.

THE DRAWINGS FIG. 1 is a schematic representation of a heating chamberconstructed in accordance with the principles of the present invention;

FIG. 2 is a view in cross section of the heating chamber of FIG. 1,taken along the line 2-2;

FIG. 2A is a schematic representation of a second embodiment of aheating chamber constructed in accordance with the principles of thepresent invention;

FIG. 3 is a functional diagram of the RF source of FIG. 1; and,

2 FIG. 4 is a perspective view of the output transformer of FIG. 3.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2 wherein a preferredembodiment of the heating chamber constructed in accordance with thepresent invention is illustrated, a plasma chamber 10 is formed within acentral passage 14 extending into a substantially gas impervious,generally electrically nonconductive or insulative envelope 12. Thesubstrate to be heated provides a central electrode 16 which extendsthrough the central passage 14 and is isolated from the chamber 10 bythe radially inward wall of the envelope 12. An electrode 18 is disposedradially outward of the envelope l2 and is separated at least in partfrom the centrally disposed electrode 16 by at least a portion of theenvelope 12, thereby defining an area within the envelope 12, i.e., atleast a portion of the chamber 10, which is disposed between theelectrodes l6 and 18.

High frequency electrical potential is applied between the electrodes 16and 18 from a suitable source such as a variable frequency and amplituderadio frequency (RF) source 20 to thereby subject the chamber defined bythe envelope 12 between the electrodes 16 and 18, to a selectable timevarying electrical field. A suitable fill tube 22 may be providedcommunicating with the chamber 10 through the envelope 12 and having avalve or other suitable closure means 24 therein to selectively controlthe gas pressure and gas constituency within the envelope 12.

With continued reference to FIGS. 1 and 2, the envelope 12 defining thechamber 10 preferably comprises an outer elongated hollow glasscylindrical member 26, an inner elongated hollow glass cylindricalmember 28, and apertured end plates 30 and 32 sealed therebetween in asuitable conventional manner. The cylindrical member 28 illustrated issubstantially coextensive with the member 26 and is disposed intelescoping relationship thereto coaxially within the member 26 todefine a chamber annular in cross section as is shown in FIG. 2.

As was previously mentioned, the substrate to be heated preferably formsthe central electrode 16. The substrate may be passed through thecentral passage 14 from a feed reel 36, over suitable guides such as therollers 38, and onto a take-up reel 40. Either or both of the rollers 38may be connected to one output terminal of the RF source, for example,by grounding the rollers 38 and one output terminal of the RF source asis illustrated in FIG. 1.

The outer electrode 18 is preferably a hollow cylindrical electricallyconductive member circumferentially disposed round at least a portion ofthe insulative member 26 and may be, for example, a metallic foilconformed to the radially outer surface of the envelope. The centralelectrode 16 preferably extends axially into the central passage 14sufficiently so that an elongated annular portion of the chamber 10 islocated bet ee t e e ec rode and1 The application of a potential fromthe RF source 20 between the electrodes 16 and 18 creates an electricfield between these electrodes, as is indicated by the lines 34 in FIG.2. The electrode configuration, i.e., the relative positions of theelectrodes and the relative dimensions thereof, cause the electric fieldto be more concentrated or dense in the vicinity of the central 3electrode 16 near the axis of the annular chamber 10.

If the intensity of the electric field is sufficient, the gas in thechamber will be excited sufficiently to create a gaseous plasma in thechamber. The plasma generally comprises highly reactive species such asions, electrons and neutral fragmented particles in highly excitedstates. Since the exciting of the gas by the electric field creates theplasma, the plasma concentration or density generally conforms to theelectric field concentration or density. Thus, the concentration ordensity of the plasma generated within the gas impervious envelope 12varies between the outer cylindrical member 26 and the inner cylindricalmember 28 in a manner related to the electric field concentration ordensity.

The relationship between the gas conditions within the envelope l2 andthe gas conditions exteriorly thereof is desirably such that the plasmamay be confined to the chamber 10. The electric potential applied to theelectrodes 16 and 18 may thus be lower and the current density will becorrespondingly less. This desirable relationship may be obtained byutilizing selected gases at predetermined pressures within the chamber10, while exposing the electrodes outside the envelope 12 to theatmosphere.

By way of example, a monatomic inert gas, such as argon or helium atatmospheric or slightly less than atmospheric pressure may be utilizedin the chamber 10. When the RF signal is applied to the electrodes 16and 18, a plasma will be more readily generated within the chamber 10than exteriorly thereof. With the potential of the RF signal applied tothe electrodes set at a value above the potential required to generate aplasma within the chamber 10, but below the potential required togenerate a plasma in the vicinity of the electrodes 16 and 18 externallyof the chamber 10, the current which flows between the electrodes 16 and18 will depend primarily upon the capacitive coupling between theelectrodes rather than on the ion flow within the plasma.

When an RF signal of sufficient amplitude is applied to the substrateforming the electrode 16 and the electrode 18, a gaseous plasma,concentrated about the inner cylindrical member 28, is generated withinthe chamber 10. The temperature of the plasma is extremely high due tothe scattering of the energy gained from the electrical field set upbetween the electrodes 16 and 18, and the temperature may be controlledwithin practical limits in direct relation to the field intensity.

The current flowing through the substrate causes resistive heating ofthe substrate apparently due to the intense magnetic and electricalfields in the plasma.

The heating of the substrate as described above results in highlyefficient use of the energy supplied by the RF source 20. Thus, therequired substrate temperature may be achieved more efficiently than byother conventional heating methods and the substrate temperature may beeasily controlled in a number of ways, for example, by controlling theamplitude ofthe RF signal, varying the diameter of the inner cylindricalmember 28, or varying the distance between the electrodes along thelength of the envelope 12 as shown in FIG. 2A. 1

As was previously described, the RF source 20 of FIG. 1 preferablysupplies a high frequency RF signal at selectable power levels to theheating apparatus of FIG. 1. As is illustrated in FIG. 3, the RF source20 may include a high power RF oscillator 42 connected to the load(e.g., the electrodes 16 and 18 of FIG. 1) through a balun transformer44. For example, the balun transformer 44 may form a portion of the tankcircuit of the oscillator 42. Maximum power transfer between theoscillator 42 and the load is thus obtained when an impedance matchexists between the oscillator tank circuit and the load impedancereflected back to the tank circuit. Since it may be desirable to varythe oscillator output power and frequency to suit the requirements ofthe heating apparatus, it may be necessary to vary the oscillator outputimpedance to retain the desired impedance match.

Impedance matching for maximum efficiency and control of the powertransfer to the load is preferably accomplished by providing a baluntransformer arrangement as is illustrated in FIG. 4.

With reference now to FIG. 4, the balun transformer 44 of FIG. 3preferably includes a primary coil 46 wound in a helical groove 47 on anelectrically insulative core 48. A secondary coil 50 is wound in ahelical groove 51 on an electrically insulative core 52 disposedcoaxially with respect to the core 48.

The coils 46 and 50 may be, for example, helically wound, hollow coppertubes generally conforming to the shapes of the helical grooves in therespective cores 48 and 52. The coil 46 may be secured to a pairofoutput terminal blocks 53 and the coil 50 may be terminated with asuitable transmission line connector 55 such as a 50 ohm connector.

The core 52 may be fixedly connected to a shaft 54 which extends througha central passage in the core 48 so that the core 48 is freely rotatableon the shaft 54. A shoulder 56 may be provided on the shaft 54 to insure a fixed spacing between the cores 48 and 52, and the end 58 of theshaft 54 may be threaded and may protrude out of the core 48 so that anut 60 may be utilized to prevent removal of the shaft 54 from thecentral passage in the core 48.

An insulative knob 62 having a position indicator 64 thereon may beconnected to the core 52 to facilitate the rotation of the core 52 andto provide an indication of the relative positions of the cores 48 and52. As the knob 62 is rotated, the secondary coil 50 moves axially alongthe core 52 varying the spacing between the coils and thereby varyingthe mutual inductance between the coils. Thus, at a particular frequencysetting, the coil spacing may be varied until an impedance match and/ora desired output power is obtained as may be indicated on a suitablewattmeter (not shown).

It should be noted that the axial spacing between the cores 48 and 52remains substantially constant as the core 52 is rotated. Thus, theminimum spacing between the coils cannot be decreased below apredetermined distance, preventing accidental arcing between the coils.Moreover, the grooves which receive the coils aid in preventing arcingby interposing a material having a high dielectric strength than that ofair at least partially between the coils 46 and 50 and between adjacentcoil windings.

While only the core 52 rotates in the embodiment of FIG. 5, it isapparent that the axial spacing between the coils may be varied in othermanners. For example, the cores 48 and 52 may be connected for axialrotation together with the cores being grooved in opposite directions,Le, a left-handed thread or groove on the core 48 and a right-handedthread or groove on the core 52.

Thus, the coils may both be movable axially in response to the rotationof the cores.

GENERAL SUMMARY OF ADVANTAGES lt is apparent from the foregoingdescription that the present invention is particularly advantageous forthe efficient and controlled electrical heating of substrates such asconductive fibers or wire. The substrate is heated both directly andindirectly, thereby making the most efficient use of the electricalpower supplying the heating energy. Moreover, the generated energy ofthe plasma acting indirectly on the substrate through thermal conductionand radiation is concentrated in the vicinity of the substrate and alsoacts indirectly on the substrate to improve the heating efficiency.

The balun transformer used in conjunction with the present inventionprovides a convenient way to maximize power transfer and to control thepower applied to the electrodes between which the plasma is generated.Moreover, the entire transformer core assembly may be easily andinexpensively constructed by conventional molding techniques and thecoils maybe constructed from commercially available tubing andcommercially available fittings. Also, accidental arcing between thetransformer windings is prevented by the novel structure of thetransformer. The present invention may be embodied in other specificforms without departing from the spirit or essential characteristicsthereof. The presently disclosed embodiment is therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims ratherthan by the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:

l. A method for heat treating substrates comprising the steps of:

providing electrically insulative means defining a gas imperviousenvelope having a central passage extending therethrough;

passing the substrate through said central passage to thereby createrelative movement between the substrate and said central passage; and,

generating a gaseous plasma within said envelope without generating aplasma in said central passage, said substrate being heated at leastindirectly by said plasma.

2. The method of claim 1 wherein said gaseous plasma is generated byapplying a high frequency electrical signal to said substrate and anelectrode disposed exteriorly of said envelope, said substrate therebybeing resistively heated by current flow therethrough.

3. The method of claim 2 wherein said electrode is cylindrical and isconcentric with said substrate, the surface area of said electrodeexceeding the surface area of said substrate whereby said plasma isconcentrated in the vicinity of said substrate.

4. The method of claim 2 wherein the current through said substrateincludes a current transverse to the axis of the envelope.

5. The method of claim 4 wherein the amplitude of the transverse currentis an order of magnitude greater than the current between the substrateand the electrode.

6. The method of claim 5 wherein the transverse current is generallyconfined to the exterior surface of the substrate and the substrate isconductively heated inwardly from the surface thereof.

1. A method for heat treating substrates comprising the steps of:providing electrically insulative means defining a gas imperviousenvelope having a central passage extending therethrough; passing thesubstrate through said central passage to thereby create relativemovement between the substrate and said central passage; and, generatinga gaseous plasma within said envelope without generating a plasma insaid central passage, said substrate being heated at least indirectly bysaid plasma.
 2. The method of claim 1 wherein said gaseous plasma isgenerated by applying a high frequency electrical signal to saidsubstrate and an electrode disposed exteriorly of said envelope, saidsubstrate thereby being resistively heated by current flow therethrough.3. The method of claim 2 wherein said electrode is cylindrical and isconcentric with said substrate, the surface area of said electrodeexceeding the surface area of said substrate whereby said plasma isconcentrated in the vicinity of said substrate.
 4. The method of claim 2wherein the current through said substrate includes a current transverseto the axis of the envelope.
 5. The method of claim 4 wherein theamplitude of the transverse current is an order of magnitude greaterthan the current between the substrate and the electrode.
 6. The methodof claim 5 wherein the transverse current is generally confined to theexterior surface of the substrate and the substrate is conductivelyheated inwardly from the surface thereof.